This invention relates to diamond abrasive particles having a multi-layer metal coating and processes for producing the same. These coated diamond abrasives find particular use in sintered metal bonded tools where the multi-layer coating aids retention of the diamond abrasives within such tools and also aids tool wear resistance.
It is well known in the art that metal coatings can improve the retention of diamond abrasive particles in the matrices of abrasive tools such as those used to saw stone and concrete. Metal coated diamond abrasives are commercially available with nickel coatings typically applied by electroless deposition. While such abrasives provide good performance, improvements are desired to reduce the premature loss of diamond abrasive particles and reduce the wear of abrasive tools.
Nickel coatings applied by electroless deposition are not chemically bound to the diamond surface. Metals which adhere to diamond surfaces more strongly are well known and include molybdenum, titanium and chromium, which are carbide formers and are typically chemically vapor-deposited or sputtered onto diamond surfaces. Examples of such coatings and processes for depositing them are disclosed in U.S. Pat. No. 3,465,916; EP-A-79/300,337.7; U.S. Reissue No. 34,133; and U.S. Pat. No. 4,063,907. Although these coatings bond more strongly to diamond surfaces than nickel coatings, these coatings are usually oxidized and can be brittle, depending on the carbide formed.
Carbide forming metal layers have been used as part of multi-layer coatings on diamond particles to aid retention within a tool matrix. U.S. Pat. No. 3,924,031 discloses a multi-layer coating for diamond particles wherein the first layer comprises an alloy with a base metal of copper, nickel or iron and a carbide-forming metal such as titanium, chromium or vanadium. This alloy layer may be over coated with another layer such as nickel by electroless or electrolytic deposition. The alloys comprise at most 30 wt. % of the carbide forming metal and, to form the carbide, the coating is heated at high temperatures after deposition by vacuum evaporation or sputtering.
U.S. Pat. No. 4,378,975 describes the use of chromium as a first coating on pelletized diamond particles which are in turn used to form abrasive bodies. A sintered copper/nickel alloy forms the outer wear-resistant coating on these pelletized particles. It is unknown whether the chromium layer forms a carbide, although the green pellets are sintered at temperatures of 900.degree. C. in forming the pelletized particles.
U.S. Pat. No. 5,024,680 describes the use of a chromium, titanium or zirconium carbide-forming layer as part of a multi-layer coating on diamond particles to aid retention within a matrix. Two carbide-forming layers are applied; one thin base layer and a thick oxidation-resistant secondary layer. A third non-carbide-forming layer applied by electroless techniques is optional. The base carbide layer of chromium, zirconium or titanium is applied by metal vapor deposition, preferably followed by heating of the coated particle to form the carbide. Chemical vapor deposition of this layer is said to provide no advantage. The secondary carbide-forming metal layer of tungsten or tantalum can be applied by CVD followed by heating of the layer to provide adequate carburization.
These procedures for applying multi-layer coatings are complex in that either metal alloys are applied as one of the layers, or three distinct layers are used. In addition, these procedures provide increased bonding strength between the diamond particles and the tool matrix through carburization of the metal coating, during which the diamond particles are exposed to high temperatures. High temperatures can cause degradation of the diamond crystal, which is detrimental to the performance of the cutting tool. Chen et al. (U.S. Pat. No. 5,024,680) recognizes this problem but provides no solution other than to avoid excess carburization.
It is desirable to apply multi-layer coatings to diamond by a simpler method which will aid its retention within the matrix of an abrasive tool without degrading the diamond particle and improve tool wear resistance. It is also desirable to provide multi-layer coatings to diamond which will enhance the wear performance of an abrasive tool.